If you are changing the pitch without also changing the playback speed, then you are resynthesizing the source sound. Sometimes it works well enough, but very often it can create some pretty noticeable artifacts. Reaper has a number of different algorithms to try. Some work better for some sources than others. I have trouble getting something I can actually use, though, when going as far as an octave and especially if it's going to be mostly clean - without effects or distortion to hide some of the weirdness.
If you change the playback speed without changing the pitch ("Preserve pitch..."), it can often have the same problems. But if you change the speed and let the pitch go with it, it's pretty much just like playing a tape or vinyl record at a different speed than it was recorded at, and that can actually be a whole lot more natural - without any of the artifacts you'd get with the other method.
So, like, if the piece is slow enough, you can whack Reaper's Playrate up to double, record your "bass" line, and then slow it back down, and it will often sound quite a bit like a real honest-to-fuck bass guitar. You lose the top octave of the sound, and with an acoustic (through a mic?) this might be a noticeable loss of the very top end zing and especially in the "air" around the sound. With a good condenser mic or piezo pickup, and recording at a higher sample rate, you can sometimes get around that issue reasonably well. It works really well with electrics, since they don't pass anything anywhere near that high anyway. It makes single coils sound like humbuckers, parallel combinations of singles sound more like singles, and humbuckers sound like darker humbuckers.
Don't know if that helps at all, but it's a fun trick I've used at times. My music is usually so slow that playing double time can sometimes actually be easier, and I've always felt like in faster music, the bass should usually play about half as much as the guitars anyway.
Now I'll get out of the way and let all the other guys line up to tell you that the only way to do it right is to buy a bass guitar.